使用 spring 反序列化 JSON：未解决的前向引用 Jackson 异常

Question

我使用 Spring 从事 API Rest 项目。 我有一个服务“CreateMateriel”，它以 JSON 为参数数据：

材料对象的 JSON

{
  "agence": 1, 
  "code": "001",
  "type": "MyType"
}

“Materiel”与“Agence”有多对一的关系。我放了一个@JsonIdentityInfo 标记来使用Agence 的Id 而不是Agence 的对象（在看到this topic 之后）

@JsonIdentityInfo(
        generator = ObjectIdGenerators.PropertyGenerator.class,
        property = "idAgence")
@JsonIdentityReference(alwaysAsId = true)
@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "agence")
private Agence agence;

但是当我在 POST /materiels 上发送 JSON 时，我遇到了这个异常：

2017-05-16 18:00:53.021  WARN 8080 --- [nio-8080-exec-8] .w.s.m.s.DefaultHandlerExceptionResolver : Failed to read HTTP message: org.springframework.http.converter.HttpMessageNotReadableException: Could not read document: Unresolved forward references for: 
 at [Source: java.io.PushbackInputStream@767dfd7a; line: 5, column: 1]Object id [1] (for fr.app.domain.Agence) at [Source: java.io.PushbackInputStream@767dfd7a; line: 2, column: 14].; nested exception is com.fasterxml.jackson.databind.deser.UnresolvedForwardReference: Unresolved forward references for: 
 at [Source: java.io.PushbackInputStream@767dfd7a; line: 5, column: 1]Object id [1] (for fr.app.domain.Agence) at [Source: java.io.PushbackInputStream@767dfd7a; line: 2, column: 14].

经过多次研究，我看到了 ObjectIdResolver 在 JsonIdentityInfo 中的使用...... 但我认为这不是最好的解决方案。这就是为什么我请求您帮助检测问题的根源。 谢谢

MaterielController.java

package fr.app.controllers;

import fr.app.domain.Materiel;
import fr.app.services.MaterielService;
import org.springframework.web.bind.annotation.*;

import javax.annotation.Resource;
import java.util.Collection;

@RestController @RequestMapping(value = "/materiels")
public class MaterielController {
    @Resource
    private MaterielService materielService;

    @RequestMapping(method = RequestMethod.POST)
    public Materiel createMateriel(@RequestBody Materiel materiel) {
        return this.materielService.createMateriel(materiel);
    }

    @RequestMapping(method = RequestMethod.GET)
    public Collection<Materiel> getAllMateriels() {
        return this.materielService.getAllMateriels();
    }

    @RequestMapping(value = "/{code}", method = RequestMethod.GET)
    public Materiel getMaterielByCode(@PathVariable(value = "code") String code) {
        //find materiel by code
        return this.materielService.getMaterielByCode(code);

    }

    @RequestMapping(value = "/{id}", method = RequestMethod.DELETE)
    public void deleteMateriel(@PathVariable(value = "id") Long id) {
        this.materielService.deleteMateriel(id);

    }

    @RequestMapping(value = "/{id}", method = RequestMethod.PUT)
    public Materiel updateMateriel(@PathVariable(value = "id") Long id, @RequestBody
            Materiel materiel) {

            materiel.setIdMateriel(id);

        return this.materielService.updateMateriel(materiel);

    }
}

Materiel.java

package fr.app.domain;

import com.fasterxml.jackson.annotation.*;

import javax.persistence.*;

@Entity
public class Materiel {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long idMateriel;

    @Column(name = "type_materiel", nullable = false)
    private String type;

    @Column(name = "code_materiel", unique = true, nullable = false)
    private String code;

    @JsonIdentityInfo(
            generator = ObjectIdGenerators.PropertyGenerator.class,
            property = "idAgence")
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "agence")
    private Agence agence;

    public Materiel() {    }

    public Materiel(String type, String code, String dateScan) {
        this.type = type;
        this.code = code;
        this.dateScan = dateScan;
    }

    public Long getIdMateriel() {
        return idMateriel;
    }

    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }

    public String getCode() {
        return code;
    }

    public void setCode(String code) {
        this.code = code;
    }

    public Agence getAgence() {
        return agence;
    }

    public void setAgence(Agence agence) {
        if(this.agence != null)
            this.agence.deleteMateriel(this);
        this.agence = agence;
        this.agence.addMateriel(this);
    }
}

MaterielService.java

package fr.app.services;

import fr.app.domain.Materiel;
import fr.app.repositories.MaterielRepository;
import org.apache.commons.collections.IteratorUtils;
import org.springframework.stereotype.Service;

import javax.annotation.Resource;
import java.util.Collection;

@Service(value = "materielService")
public class MaterielServiceImpl implements MaterielService {
    @Resource
    private MaterielRepository materielRepository;

    ...

    @Override
    public Materiel createMateriel(Materiel materiel) {
        return this.materielRepository.save(materiel);
    }

   ...
}

Agence.java

package fr.app.domain;

import com.fasterxml.jackson.annotation.*;

import javax.persistence.*;
import java.util.HashSet;
import java.util.Set;

@Entity
public class Agence {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long idAgence;

    @Column(name = "nom",unique = true, nullable = false)
    private String nom;

    @OneToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL, mappedBy = "agence")
    private Set<Materiel> materiels = new HashSet<Materiel>();

    public Agence() { }

    public Agence(String nom) {
        this.nom = nom;
    }

    public Long getIdAgence() {
        return idAgence;
    }

    public String getNom() {
        return nom;
    }

    public void setNom(String nom) {
        this.nom = nom;
    }

    public Set<Materiel> getMateriels() {
        return materiels;
    }

    public void setMateriels(Set<Materiel> materiels) {
        this.materiels = materiels;
    }

    public void addMateriel(Materiel materiel) {
        this.materiels.add(materiel);
    }

    public void deleteMateriel(Materiel materiel) {
        this.materiels.remove(materiel);
    }

}

pom.xml

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-data-jpa</artifactId>
    </dependency>
    <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
        <version>5.1.6</version>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
    <dependency>
        <groupId>commons-collections</groupId>
        <artifactId>commons-collections</artifactId>
        <version>3.0</version>
    </dependency>
    <dependency>
        <groupId>org.hibernate</groupId>
        <artifactId>hibernate-core</artifactId>
        <version>5.2.9.Final</version>
    </dependency>
    <dependency>
        <groupId>org.hibernate</groupId>
        <artifactId>hibernate-entitymanager</artifactId>
        <version>5.2.9.Final</version>
    </dependency>
    <dependency>
        <groupId>io.springfox</groupId>
        <artifactId>springfox-swagger2</artifactId>
        <version>2.6.1</version>
    </dependency>
    <dependency>
        <groupId>io.springfox</groupId>
        <artifactId>springfox-swagger-ui</artifactId>
        <version>2.2.2</version>
        <scope>compile</scope>
    </dependency>

    <!-- Needed for JSON View -->
    <dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
        <artifactId>jackson-databind</artifactId>
        <version>2.8.6</version>
    </dependency>
</dependencies>

Answer 1

好的，我找到了问题。

最后我需要添加解析器。所以我找到here 一个实现ObjectIDResolver 的例子：

import com.fasterxml.jackson.annotation.ObjectIdGenerator;
import com.fasterxml.jackson.annotation.ObjectIdResolver;

import javax.persistence.EntityManager;

/**
 * @author fta on 20.12.15.
 */

public class EntityIdResolver
        implements ObjectIdResolver {

    private EntityManager entityManager;

    public EntityIdResolver(
            final EntityManager entityManager) {

        this.entityManager = entityManager;

    }

    @Override
    public void bindItem(
            final ObjectIdGenerator.IdKey id,
            final Object pojo) {

    }

    @Override
    public Object resolveId(final ObjectIdGenerator.IdKey id) {

        return this.entityManager.find(id.scope, id.key);
    }

    @Override
    public ObjectIdResolver newForDeserialization(final Object context) {

        return this;
    }

    @Override
    public boolean canUseFor(final ObjectIdResolver resolverType) {

        return false;
    }

}

接下来我添加@JsonIdentityInfo:

@Entity
@JsonIdentityInfo(
        generator = ObjectIdGenerators.PropertyGenerator.class,
        property = "idAgence",
        resolver = EntityIdResolver.class,
        scope=Agence.class)
public class Agence {
    // ... 
}

Answer 2

由于您的一对多关系，您似乎有一个循环对象图。对于那些你可以使用JSOG serializer plugin for Jackson的人。

据说它可以处理循环对象图的序列化和反序列化（自 Jackson 2.5.1 起）

@JsonIdentityInfo(generator=JSOGGenerator.class)
public class Person {
    String name;
    Person secretSanta;
}

Answer 3

您有两个域：Materiel 和 Agence。当 Jackson 尝试反序列化 Materiel 时，它调用方法 getAgence。此方法返回一个 Agence 对象，该对象也将被 Jackson 反序列化。当 Jackson 调用方法 getMeteriels（来自 Agence）时，它会返回一组也将反序列化的材料。问题是 Jackson 将尝试再次反序列化 Set 中的每个 Materiel。同样的问题here。

Answer 4

对于您的最后一个例外 (utf-8)，您可以更改控制器的签名，如下所示：

@RequestMapping(method = RequestMethod.POST, produces = MediaType.APPLICATION_JSON_VALUE, consumes = MediaType.APPLICATION_JSON_VALUE)
public ResponseEntity<Bean> create(HttpServletRequest  request) throws IOException {
    // getting the posted value
    String body = CharStreams.toString(request.getReader());
    Bean bean = new ObjectMapper().readValue(body, service.getBeanClass());
    bean.setId(null);
    Bean saved = service.save(bean);

    URI location = ServletUriComponentsBuilder.fromCurrentRequest().path("/{id}").buildAndExpand(saved.getId()).toUri();
    return ResponseEntity.created(location).body(saved);
}

使用此解决方案，您可以完全控制映射。

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